This project is concerned with investigating the in vivo differentiation of the thymus-dependent lymphocyte and its interaction in vitro with foreign cells by means of correlated techniques of conventional electron-microscopy, freeze-fracturing and ultrastructural cytochemistry. In a suspension of DBA/2 P-815-X2 mastocytoma cells (target) and sensitized C57 BL/6 lymphocytes (Killers), the clusters of adhering cells will be separated by velocity sedimentation and analyzed with the freeze-fracturing and freeze-etching techniques in order to establish the membrane events which accompany the interaction between killer and target. Changes in internal organization of the plasma membrane of the target cells will be investigated at different time intervals following the interaction with the killer. The vascular permeability of stimulated lymph nodes will be studied using ultrastructural tracer techniques.